This invention is related to the stripping of nickel, nickel alloys and nickel reaction products from a variety of metal substrates, and, more particularly, to a novel stripping composition and accompanying method for the selective removal of these nickel coatings without damaging the underlying substrate.
Selective metal stripping is one of the most common steps in many industrial manufacturing processes. Usually, the stripping is part of either a general overhaul, involving the refurbishing of a particular coating, the reclaiming of a defectively plated part, or the recovery of the metal coating. The overhauling process becomes economically feasible when the specific product to be stripped is particularly valuable, such as in the case of high performance aircraft engine components. Also, defective plating will usually occur in a certain percentage of plated parts, resulting from imperfections in the basis metal, improper cleaning, excessive porosity of the substrate which leads to bleeding out of various cleaning and plating solutions, impurities, and human error.
One area of particular interest to industry is the selective stripping of electrolytic nickel and low phosphorous electroless nickel coatings from mild ferrous (e.g., steel) substrates. For this process it is necessary to strip substantially all, e.g., greater than 99%, of the coating because upon subsequent reprocessing lamination, blistering, skip plating, patterned appearance and dimensional changes may occur. Acidic strippers containing phosphoric acid are rapid and don't attack the steel substrate; however, during stripping, after about 95% of the nickel is removed, and electric potential is formed between the steel surface and the nickel remaining on the surface and the part becomes passive and stripping ceases. A two-step procedure is usually employed wherein after the phosphoric acid stripping, an alkaline stripper is used to strip the remaining nickel coating. The two-step procedure is inefficient and waste disposal problems are increased because of the alkaline stripper.
Another important industrial application is the selective stripping of reaction product coatings from nickel-base jet engine parts. These coatings are generally known as nickel sulfidation products and must be removed to enable inspection of the part.
The particular method of stripping will usually depend upon the metal (or metals) to be stripped, the substrate material of the basic part, waste disposal requirements and profitability. The prior art has employed a variety of both chemical and mechanical methods in an effort to selectively remove nickel and nickel compound coatings from the underlying metal substrate, however, they have proven unsatisfactory for a variety of reasons. Damage to precision machined parts such as jet engine stator assemblies, particularly the large dimensional changes which can result on the metal substrate, as well as pit formation in the base metal caused by pinholes in the hard coating are frequent results of using pressure blasting, abrasive tool methods and electrolytic stripping.
Low metal stripping rates, general ineffectiveness at low operating temperatures, the handling and disposal of toxic cyanide containing solutions, and the environmental disposal of the spent fluids are problems arising from known chemical methods. For example, U.S. Pat. No. 3,365,401 discloses a bath for stripping nickel from base metal objects comprising an aqueous solution of a nitro-substituted mononuclear carbocyclic aromatic compound, a complexing agent for nickel ions, ammonium ions to maintain the pH above about 6.8 and a sulfur compound yielding in the aqueous solution sulfur ions in a -2 oxidation state. Another nickel chemical stripper is shown in U.S. Pat. No. 3,717,520 and is an alkaline solution comprising a nitro-substituted aromatic compound, elemental sulfur, alkali phosphate, alkali chloride, alkylene polyamine and a corrosion attack inhibitor such as an alkali metal nitrite. Hydrogen peroxide-sulfuric acid mixtures are disclosed in U.S. Pat. Nos. 3,293,093; 4,130,455 and 4,174,253 for the etching of copper on printed circuit boards. The use of sulfuric acid in combination with hydrogen peroxide proved unacceptable for stripping nickel from mild steel as shown in the examples.
Accordingly, it is an object of this invention to formulate an effective composition and accompanying method for the selective stripping of nickel and nickel compound coatings from metal substrates.
Another object of the invention is to provide an effective stripping composition which may be easily and safely treated for waste disposal.